1. Field of the Invention
The present invention relates generally to oxygen sensing devices and more particularly to such devices utilizing an electrolytic wafer and intended for use in the exhaust path of an internal combustion engine.
2. Prior Art
In recent years, it has become known to measure the content of oxygen in combustion gases by the use of an electrochemical oxygen sensing device which is exposed to the exhaust gases in an internal combustion engine. The prior art devices have been of two general types, the first of these being referred to as the thimble type, examples of which are disclosed in U.S. Pat. Nos. 3,960,693 and 3,978,006; and the second of these being known as a disc or wafer type, examples of which are shown in U.S. Pat. Nos. 3,909,385 and 3,940,327. Sensors of both types utilize a solid electrolyte such as zirconium oxide (ZrO.sub.2) which generates a voltage between the electrodes of the sensor when there is a difference in oxygen partial pressures between the exhaust gas side of the electrolyte and the reference, or air, side of the electrolyte. Because of this, it is absolutely necessary to maintain a good seal along the edges of the electrolyte so that the exhaust gas and reference gas are kept separated. The required seal has been accomplished in prior art devices in various ways. In U.S. Pat. No. 3,909,385, a wafer of electrolyte material is sealed in a metal housing by a hermetic seal formed from a metal oxide frit which wets both the surface of the wafer and the pre-oxidized metal housing. Upon heating, the frit softens to form the necessary seal. In U.S. Pat. No. 3,940,327, means for holding an electrolyte wafer in contact with the flange of a metallic housing is described wherein a ceramic insulating sleeve is used in connection with a high temperature fiber gasket to provide a hermetic seal to prevent leakage of exhaust gases around the wafer. The principal seal is thus provided by the high temperature ceramic fiber gasket.
The present invention is intended to provide an improved and alternate method of forming a gas-tight seal in an oxygen sensing device of the type mentioned.